THE HORSESHOE CRAB IS ONE MODEL EMPLOYED TO STUDY THE INVERTEBRATE ANTIMICROBIAL DEFENSES THAT INCLUDE: THE CLOTTING OF CIRCULATING BLOOD CELLS, BINDING TO SOLUBLE LECTINS, AND NEUTRALIZATION OF TOXINS BY INTERACTION WITH PEPTIDE ANTIBIOTICS. OUR LABORATORY HAS A COLLECTION OF PIGMENTED AND NON-PIGMENTED BACTERIA ISOLATED FROM A VARIETY OF MARINE ENVIRONMENTS. WE ARE STUDYING THE ROLES PLAYED BY THESE BACTERIA AND THEIR PIGMENTS AS PROTECTIVE MECHANISMS FOR MARINE INVERTEBRATES THAT FREQUENTLY HARBOR THESE BACTERIA ON THEIR SURFACES AND IN THEIR TISSUES. ANTIBIOTIC SCREENING HAS SHOWN THAT SOME OF THE ISOLATES PRODUCE SECONDARY METABOLITES HAVING ANTIBIOTIC ACTIVITY. IT HAS BEEN NOTED THAT PIGMENTED MARINE BACTERIA ARE MORE RESISTANT TO HEAVY METALS AND ANTIBIOTICS. PRELIMINARY EXPERIMENTS IN COLLABORATION WITH DR. PETER SMITH, USING THE NATIONAL VIBRATING PROBE FACILITY, HAVE DEMONSTRATED THAT BACTERIAL PIGMENTS CAN ALTER THE ION FLUXES IN CULTURED CELLS IN THE PRESENCE OF PEROXIDES. FURTHER CHARACTERIZATION OF THE EFFECT OF EXTRACTED AND PURIFIED PIGMENTS WILL ESTABLISH THE CONNECTION BETWEEN REACTIVE OXYGEN SCAVENGERS AND CELL PROTECTION.